Photodetectors, vertical shift registers, and horizontal shift registers all have a maximum amount of photo-generated charge each can store. Excess charge spills or “blooms” into neighboring components when the photo-generated charge exceeds the maximum amount. Blooming is a serious problem in image sensors because blooming corrupts the image output from the image sensor.
A camera having an image sensor operating in either a sub-sampling mode or a focus mode can transfer excess photo-generated charge to a horizontal shift register. A sub-sampling mode enables a camera to operate with a fast frame rate having less resolution, such as a video preview mode. For example, a “5x” sub-sampling in the y-direction reads out only one line out of every five lines. The image sensor inside the camera dumps four successive lines of charge from the vertical shift registers into the horizontal shift register and removes the charge from the horizontal shift register before the transfer of the fifth line of charge. If the amount of photo-generated charge in the four lines of charge exceeds the capacity of the horizontal shift register, the excess charge spills back into the vertical shift registers and creates a “blooming” image.
A focus-mode enables the image sensor to read out only a portion of the image by dumping the leading and trailing portions of the image. The leading or the trailing portions can include many lines of charge. Blooming occurs when the total amount of charge dumped into the horizontal shift register exceeds the capacity of the horizontal shift register. Blooming corrupts the image and adversely affects the auto-focusing functionality of the camera.
U.S. Pat. No. 5,902,995 discloses a solution to the problem of blooming in the horizontal shift register. FIG. 1 depicts a top view of a portion of an image sensor disclosed in U.S. Pat. No. 5,902,995. Image sensor 100 has array of photodetectors 102 that convert incident light into photo-generated charge. Each pixel has four vertical gates 104, 106, 108, 110 that are used to shift photo-generated charge through the vertical shift register regions 111. Each horizontal shift register region 112 also has four gates 114, 116, 118, 120 that are used to sequentially shift the photo-generated charge through the horizontal shift register region.
Photo-generated charge that is transferred to the vertical shift register regions 111 is shifted vertically down through a channel 124 in each vertical shift register region 111, and then transferred to the horizontal shift register region 112. An overflow barrier region 128 extends across and is embedded below channel 126. An overflow drain region 130 is connected to channel 126 of horizontal shift register region 112. Overflow drain region 130 discharges any excess charge that exceeds the barrier potential.
However, due to relatively high sheet resistance of overflow drain region 130, having a long stripe of an overflow drain region 130 extending across the entire horizontal shift register region 112 produces a voltage drop from the center of the stripe to the point where the stripe has a conductive contact that connects to a power supply. This voltage drop degrades the effectiveness of the overflow drain performance.
To overcome the drop in voltage, the overflow drain under each horizontal gate should be connected to a metal bus to eliminate the voltage drop caused by the conductive contact. However, this solution is not feasible when the length of a horizontal gate (L1 or L2) is too small to accommodate a conductive contact.